Add some more Eshell history tests

Add some more Eshell history tests

* test/lisp/eshell/em-hist-tests.el (em-hist-test/check-history-file):
New function.  Use it throughout this file.
(em-hist-test/history-append): Rename to...
(em-hist-test/write-history/append): ... this.
(em-hist-test/history-read): Rename to...
(em-hist-test/write-history/overwrite): ... this.
(em-hist-test/write-history/append-multiple-eshells)
(em-hist-test/write-history/overwrite-multiple-shells): New tests.
(em-hist-test/write-history/read-only): Check the resulting history.